In the rat pituitary tumor cell line GH3, the expression of the prolactin gene is stimulated by the sex steroid 17-beta-estradiol and the hypothalamic factor thyrotropin releasing hormone (TRH). The expression of the growth hormone gene in this tumor line is stimulated by corticosteroids and thyroid hormones. At present, the quantitative changes in mRNA production coupled to hormonal induction are most readily explained by supposing that primary control lies with transcription. We will study the primary effect of hormones on gene regulation in these tumor cells by making a careful analysis of transcriptional units, their instantaneous rates of transcription, the formation of mRNAs from nuclear precursors and the metabolic fate of these mRNAs. These experiments utilize cDNA clones specific for growth hormone and prolactin RNA sequences. The chromosomal segments of these genes will also be isolated and cloned to facilitate a detailed analysis of transcriptional unit organization. The transcriptional units will be related to the sequences at the capped 5' end and poly-adenylated 3' end of nuclear and cytoplasmic RNA. These studies will provide criteria for the development of an authentic in vitro system of transcription, and will contribute to an understanding of regulation of gene expression in a hormone-responsive tumor cell line.